In sealing such gaps, for example, between a heat exchanger block and an outer housing of the heat exchanger, there is the problem that the stream of gas flowing through the heat exchanger block must not be allowed to partially bypass the block through gaps, despite the fact that thermally induced expansions and displacements occur and make conventional sealing difficult. One state-of-the-art solution involves the use of brush seals, although these are disadvantageous due to high manufacturing cost. Furthermore, they suffer from premature wear and display a certain amount of permeability to the gas stream.